Parts for machine structures, differential gears, transmission gears, toothed carburized shafts, and other gears sometimes break at the tooth bases after low cycle fatigue (fatigue in region of several hundreds to several thousands of cycles) due to sudden starts and sudden stops of the vehicles. In particular, greater improvement of the low cycle fatigue strength is being sought for differential gears and for transmission gears.
In the past, as the steel material for the above steel parts, JIS SCr420, SCM420, and other around C: 0.2% case hardened steel has been used to secure toughness of the core. “Carburizing and quenching” and around 150° C. low temperature tempering are used to form an around C: 0.8% tempered martensite structure on the surface and to improve the high cycle bending fatigue strength and wear resistance.
As a steel part raised in low cycle bending fatigue strength, Patent document 1 discloses a carburized part which contains C: 0.1 to 0.3% and B: 0.005% or less, restricts Si to 0.3% or less and P to 0.03% or less, and gives a core hardness of HV350 or more.
Patent document 2 discloses case hardened steel which restricts C to 0.15 to 0.3%, Si to 0.5% or less, and P to 0.01% or less and makes the sum of the plastic deformation resistance and grain boundary strength, calculated from the composition, to a certain value or more so as to raise the low cycle fatigue strength.
Patent document 3 discloses a carburized gear which has an excellent low cycle fatigue strength which restricts C to 0.1 to 0.3%, B to 0.001 to 0.005%, Si to 0.5% or less, and P to 0.03% or less, and has a core hardness of the tooth roots of HV300 or more.
Patent document 4 discloses a carburized part which restricts C to 0.15 to 0.3%, B to 0.0003 to 0.005%, Si to 0.03 to 0.25%, and P to 0.02% or less and makes the values related to the core hardness which are calculated from the composition of ingredients a certain value or more so as to raise the low cycle impact fatigue characteristic.
Patent document 5 discloses carbonitrided bearing steel which is comprised of C: 0.1 to 0.4%, Si: 1.0% or less, Mn: over 1.5 to 3%, P: 0.03% or less, S: 0.03% or less, Cr: 0.3 to 2.5%, Al: 0.005 to 0.050%, Ti: 0.003% or less, O: 0.0015% or less, N: 0.025% or less, and a balance of unavoidable impurities and Fe and which is carbonitrided or then treated by secondary quenching and tempering to give a surface hardness of 58HRC or more and an amount of surface residual austenite of 20 to 50%.
Patent document 6 discloses a carburized quenched steel material which is excellent in low cycle fatigue characteristics which contains C: 0.1 to 0.4%, Si: 0.02 to 1.3%, Mn: 0.3 to 1.8%, S: 0.001 to 0.15%, Al: 0.001 to 0.05%, N: 0.003 to 0.020%, P: 0.025% or less, and O: 0.0025% or less, which further contains one or more types of Cr: 1.8% or less, Mo: 1.5% or less, Ni: 3.5% or less, B: 0.006% or less, V: 0.5% or less, Nb: 0.04% or less, and Ti: 0.2% or less, and which has a balance of iron and unavoidable impurities, which steel material has a projected core hardness Hp-core defined by the following formula (1) (=Hcore/(1−t/r) [Hcore: core hardness, t: effective hardened layer depth, r: radius of broken portion or half of thickness of broken portion]) of HV390 or more.
Patent document 7 discloses case hardened steel which is excellent in surface fatigue strength of a hydrogen embrittlement type which is comprised of C: 0.1 to 0.4%. Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.03% or less, Cr: 0.3 to 2.5%, Mo: 0.1 to 2.0%, V: 0.1 to 2.0%, Al: 0.050% or less, O: 0.0015% or less, N: 0.025% or less, and V+Mo: 0.4 to 3.0%, which has a balance of Fe and unavoidable impurities, and which is carburized, quenched, and tempered, which steel has a surface C concentration after quenching of 0.6 to 1.2%, has a surface hardness of HRC58 to less than 64, and has a ratio of number of fine V-based carbide grains of a grain size of less than 100 nm in the V-based carbide grains which are dispersed and precipitated at the surface of 80% or more.
However, in each carburized steel part, the low cycle bending fatigue strength does not reach the currently sought level of the low cycle bending fatigue strength.